Discharge lamp with magnetic stabilization



Dec. 3, EGEL DISCHARGE LAMP WITH MAQNETIC STABILIZATION b Filed Dec. 14, 1961 FQZ.

ll U m lnve'n tow; l iovs t SchLegeL pg 6 His A t' tovneg United States Patent Ofiice 3,ll3,23 i- Patented Dec. 3, 163

3,113,234 DISCHARGE LAMP WITH MAGNETIC STABiLlZATlUN Horst Sehlegel, lierlin-Kladow, Germany, assignor to Patent-Treuhand-Geselisehaft iiir eieldrische Gliihlampen rn.b.H.,

Filed Dec. 14, 1961, Ser. No. 159,309 Claims priority, application Germany Dec. 27, 1960 8 Claims. (Cl. 3l3155) The present invention relates to high-pressure discharge lamps wherein the discharge are is stabilized by a magnetic field.

In general, the use of a magnetic field to influence an electric discharge are is already well known. The required magnetic field may be produced between the poles of a permanent magnet or electromagnet placed outside the discharge lamp. The magnets as well as the poles may be differently shaped and arranged above the discharge lamp as well as aside of the lamp or partly within recesses in the envelope near the electrodes. For the special case of horizontal burning or operation of the discharge lamp, it is also known to place one of the current inleads in parallel with the discharge or both current inleads in parallel with each other and upward vertical to the lamp. It is also known to use coils outside the discharge lamp and connected in series with it, the magnetic field of the coils acting on the arc oppositely to the convection currents occurring in the lamp.

The known arrangements for influencing magnetically the discharge are have the disadvantage that in case of any alteration in position of the lamp, the adjustment of the magnetic means must also be altered. Devices have been proposed for automatically adjusting the position of the magnets, for instance by means of a pendulum, but they are rather cumbersome and in general unsatisfac' tory.

The object of the invention is to provide improved magnetic stabilization particularly suitable for high pressure short are gap discharge lamps wherein the inter electrode distance or gap does not exceed the inner diameter of the envelope.

The present invention provides a simple and elegant solution to the problem of magnetically stabilizing the discharge arc of electric high-pressure discharge lamps, in particular short are gap lamps. It extends the range of permissible burning positions of the lamp to inclinations of :120 from the vertical. It is characterized by a special construction of the electrodes to produce the magnetic field, at least one of the electrodes being constructed as a coil surrounding an emitting portion, the axis of the coil lying in the direction of the lamp axis.

Electrodes designed in this manner are quite different from electrodes utilizing the hollow cathode efiect. Nor are they comparable to the usual coiled electrodes which may also be arranged along the direction of the discharge lamp axis but wherein the windings are short-circuited, as by mutual contact of the windings or by a core of electron-emitting material such as thoriated tungsten within the coil. In electrodes according to the invention, the turns or windings of the coil do not touch. In operation of the lamp, current flowing through the coil produces an axially symmetrical field whose force lines extend generally parallel to the lamp axis. The stabilizing efiect of the field is so great that the arc burns stably not only in the vertical position but also in any desired position of the lamp inclined by as much as 120 to the vertical.

In inclined or horizontal burning of the lamp, the stabilization achieved by the electrodes according to the invention prevents upward deflection of the discharge are by convection currents as occurs in non-stabilized arcs.

The risk of local overheating of the bulb wall is thereby avoided. Also no diiliculties occur when the lamp in used in a casing with optical elements because the arc burns stably and the light centre remains in the lamp axis. As already mentioned, it is well known to bring the are back into normal position by means of a magnetic field having its force lines extending normally to the plane including the direction of lift and the arc axis. However heretofore such field was generally produced by external magnets and the arrangement was subject to the great drawback that the magnets must always be adjusted exactly for a definite lamp position. In the case of search lights, for instance, such quick alteration may not be possible or practical. The arrangement according to the invention overcomes this drawback.

In determining the required axially symmetrical magnetic field for the lamp, it was assumed by analogy to electron optics that a field symmetrical with respect to an axis of rotation has focussing properties relative to such axis in accordance with Buschs theorem. It was initially supposed that the arc could be influenced in similar manner across the plasma electrons as .in the case of electrons in a vacuum. The electrodes of the lamp have been constructed in such a manner that the force lines of the field produced by them are at least nearly symmetrical with respect to the axis of rotation, that they are parallel to the axis in its neighborhood, and that they converge toward the coil electrode at a greater distance from the axis. However it was found that in spite of a certain similarity in the behavior of the arc to that of a free electron ray, it is not sufficient to take equations applicable to the latter, and for an exact solution of the problem it is necessary to apply plasma dynamics. It is also desirable, particularly if the lamps are intended for projection purposes, to stabilize the arc against rotation or turning round the axis in order to avoid instability of the arc and shift of the light centre.

Calculations confirmed by experiments have shown that for operation on direct current, it is advantageous if one of the electrodes only, preferably the cathode, is made as a coil and the other electrode is made as a rod. The coil may be cylindrical or conical and may have a cross section which diifers to some extent from the circular form. In a preferred embodiment, the cathode is formed as a coil or helix provided with a tip on the end facing the discharge and the anode is rod-shaped with a conical shoulder at its end facing the discharge. Both the electrodes may, however, also be made as coils and this would be the case in a lamp intended for operation on alternating current.

A lamp according to the invention is illustrated in the drawing wherein:

FIG. 1 is a side elevation, sectioned through the bulb, of a high pressure short arc gap lamp constituting a pre ferred embodiment; and

'FIG. 2 is a longitudinal sectional view through the cathode of the lamp.

The envelope or bulb 1 of the lamp shown in FIG. 1 is made of quartz. The anode 2 consists of a solid tungsten rod supported on an inlead 3 sealed through the stem 4. Opposite the anode there is provided at an intermediate gap or distance D of approximately 10 mm., the coil cathode 5 supported from an inlead 6 sealed through stem 7. The stems 4, 7 and the respective bases therefor 8, 9 are conventional for high pressure lamps. The lamp is filled with xenon at a filling pressure between 1 and 10 atmospheres, suitably about 5 atmospheres. The lamp operates on 110 volts DC. and the operating current lies between 65 and amperes at an input of 2 to 3 kilowatts. The lamp may be operated vertically with the anode uppermost and in any inclined position deviating up to :120 therefrom, that is to the extreme inclination wherein the anode is depressed 30 below horizontal.

Cathode 5, as shown in greater detail in FIG. 2, has a magnetic field producing coil portion 1% forming of l'ectively an extension of the tungsten wire inlead 6. Of course the coil may also be separately formed and attached to the inlead, as by welding. Wire 6 has a diam ter of 3 mm. and the inner diameter of the coil portion 14) consisting of 4 turns or windings ll amounts to 19 mm. The turns 11 are spaced apart a distance of 0.5 mm. from each other. The inner end of wire s is extended beyond the front turn of coil ill and wound tight ly around a tungsten rod 12 located on the lamp axis. The several turns 13 of wire 6 wound around tungsten rod 12 serve only to support the rod and have no fluence on the magnetic field because these turns are short circuited by the tungsten rod and current does not circulate through them. The tungsten rod is the elec tron emitting portion proper and may include a sub stance such as thorium to improve its electron emission. It has a conical tip 14 projecting forward of the front turn of coil ill in the direction of the anode, the elon gated side faces of the tip forming together an angle of about 45"v Coil it] thus provides both mechanical support and circuit continuity from the inlead 6 to the electron emitting portion proper of the cathode consisting of tungsten rod 12. The lamp current is supplied to the electron emitting portion proper through coil 1i! and, as a result of the helical current flow or circulation therethrough, produces an axial magnetic field which stabilizes the arc in the inter-electrode gap.

The lamp has a brightness of about 16 ksb. or 16,00C candles/sq. cm. and a luminous eificiency of 3.6 candela/W or of about 35 lumens per watt. The magnetic field present in the lamp amounts to about 10-40 oersteds and the field is stronger near the cathode than near the anode.

The specific length and diameter of the magnetic fieldproducing coil, the number of turns, and the extent to which the cathode tip projects beyond the first turn will depend on various constructional features and operating data of the lamp including the inter-electrode gap or distance and the operating current. The invention is particularly useful in so-called compact source lamps where the inter-electrode gap is less than the inner diameter of the lamp envelope or bulb. In general, the magnetic field-producing coil will have a length and inner diameter not more than 3 nor less than /3 times the inter-electrode gap. In certain cases, the coil may be formed with two or more layers of turns or windings.

Magnetic stabilization of the discharge are by the coiled electrode construction according to the invention is applicable also in lamps filled with other rare gases or metal vapours. It may be used in lamps having operating pressures higher than atmospheres in which case the magnetic field produced in the lamp may amount to 5 to 100 oersteds.

The usefulness of lamps according to the invention is greatly enhanced by reason of the freedom achieved for the lamps burning position and the elimination of external stabilizing means. This is particularly advantageous in certain devices such as projectors or special projector lamps utilizing reflectors, for instance parabolic reflectors. Since the lamp may be operated horizontally, deeper reflectors having greater curvature may be used, thereby achieving higher efficiency and better utilization of the light source.

By means of coiled electrode magnetic stabilization it is possible, within the realm of short are gap lamps, to increase the inter-electrode distance and the arc length. In projection use, this permits a more uniform illumination of rectangular image apertures. The lamp is also particularly suitable for use in optical devices designed for greater are dimensions, for instance in maritime signalling devices which hitherto have used carbon arcs only.

The preferred embodiment of the invention which has been illustrated and specifically described is intended by 'way of example only, and the scope of the invention proper is to be determined by the appended claims.

Vhat I claim as new and desire to secure by Letters Patent of the United States is:

l. A magnetically stabilized high pressure discharge lamp comprising an envelope containing an ionizable filling and having a pair of electrodes supported on inleads sealed therein, at least one of said electrodes having an emitting portion located on the lamp axis and surrounded by a coil providing circuit continuity from the inlead to the emitting portion, the turns of said coil generally surrounding said emitting portion but being spaced therefrom and from each other whereby current ilowing to said emitting portion circulates through said coil and produces an axial magnetic field in said lamp.

2. A lamp as defined in claim 1 wherein the emitting portion of said one electrode comprises a tungsten rod having a tip projecting forward of the front turn of said coil in the direction of the anode.

3. A magnetically stabilized short are gap high pressure discharge lamp comprising an envelope containing an ionizable filling and having a pair of electrodes supported on inleads sealed therein, said electrodes together defining an arc gap not longer than the inner diameter of the envelope, at least one of said electrodes having an emitting portion located on the lamp axis and surrounded by a coil providing mechanical support and circuit continuity from the inlead to the emitting portion, the turns of said coil being spaced from said emitting portion and from each other whereby current flowing to said emitting portion circulates through said coil and produces an axial magnetic field in said lamp.

4. A lamp as defined in claim 3 wherein the emitting portion of said one electrode comprises a tungsten rod having a tip projecting forward of the front turn of said coil in the direction of the anode.

5. A magnetically stabilized short arc gap high pressure discharge lamp comprising an envelope containing an ionizable filling and having a pair of electrodes supported on inleads sealed therein, said electrodes together defining an arc gap not longer than the inner diameter of said envelope, at least one of said electrodes comprising an open coil surrounding and spaced from an emitting portion located on the lamp axis, said coil forming an extension of the inlead and having its turns spaced apart from each other, the extended to support said emitting portion, the inner diameter and the axial length of said coil being in the range of 3 to /3 times the arc gap distance, the axis of said coil lying substantially in the inter-electrode axis of said lamp whereby current flowing through said coil to said emitting portion to support the discharge simultaneously produces an axial magnetic field in said lamp for stabilizing the discharge are.

6. A magnetically stabilized short are gap high pressure discharge lamp comprising an envelope containing an ionizable filling and having a pair of electrodes supported on inleads sealed therein, said electrodes together defining an arc gap not longer than the inner diameter of the envelope, at least one of said electrodes comprising an open coil surrounding and spaced from a tungsten rod located on the lamp axis, the coil forming an extension of the inlead for said electrode and having its turns spaced apart from each other, the inner end of the coil being extended and wrapped around the tungsten rod to support it, said rod havinga tip projecting forward of the front turn of the coil in the direction of the anode, the inner diameter and t e axial length of the coil being in the range of 3 to /3 times the arc gap inner end of the coil being netic field of 5 to 100 oersteds as a. result of circulation of the lamp current therethrough.

distance, the axis of the coil lying substantially in the inter-electrode axis or the lamp whereby current flowing through the coil to the tungsten rod to support the discharge simultaneously produces an axial magnetic field f n s (Iit d in the file of this patent in the lamp for stabilizing the discharge arc. 5 v

7. A lamp as defined in claim 6 wherein the other UNI'TED STATES PATENTS I electrode is rod-shaped. ,8 ,187 Rlves May 12, 1931 8. A lamp as defined in claim 6 wherein the coil di- 17, 38 Francis Oct. 8, 1940 mensions determine in the inter-electrode space a mag- 43, 27 Buckingham Feb. 27, 1951 

1. A MAGNETICALLY STABILIZED HIGH PRESSURE DISCHARGE LAMP COMPRISING AN ENVELOPE CONTAINING AN IONIZABLE FILLING AND HAVING A PAIR OF ELECTRODES SUPPORTED ON INLEADS SEALED THEREIN, AT LEAST ONE OF SAID ELECTRODES HAVING AN EMITTING PORTION LOCATED ON THE LAMP AXIS AND SURROUNDED BY A COIL PROVIDING CIRCUIT CONTINUITY FROM THE INLEAD TO THE EMITTING PORTION, THE TURNS OF SAID COIL GENERALLY SURROUNDING SAID EMITTING PORTION BUT BEING SPACED THEREFROM AND FROM EACH OTHER WHEREBY CURRENT FLOWING TO SAID EMITTING PORTION CIRCULATES THROUGH SAID COIL AND PRODUCES AN AXIAL MAGNETIC FIELD IN SAID LAMP. 